Generally, this invention relates to improved process and apparatus which utilize a sorbent, or sorbent particles, for removing at least NO.sub.x, nitrogen oxides (NO, NO.sub.2, and N.sub.2 O.sub.4), from gas.
More particularly, this invention is an improvement in the process and apparatus disclosed in U.S. Pat. No. 4,798,711 entitled PROCESSES FOR REMOVING NITROGEN OXIDES, SULFUR OXIDES AND HYDROGEN SULFIDE FROM GAS STREAMS, issued Jan. 17, 1989, Lewis G. Neal, et al. inventors, and assigned to NOXSO Corporation, the same assignee as the present invention; this patent is hereby incorporated herein by reference, and this prior art process is referred to in the art and hereinafter and in the appended claims as the NOXSO Process. The NOXSO Process is illustrated in FIG. 3 of the 4,798,711 patent and, for convenience of reference, FIG. 3 of this patent is reproduced in the present drawings as FIG. 1. Referring to such FIG. 1, there is shown a flue gas stream 12 containing both NO.sub.x and SO.sub.x, sulfur oxides (SO.sub.2 and SO.sub.3) from, for example, a coal-fired or oil-fired power plant (not shown) which flue gas stream 12 is passed through a fluid bed adsorber 14 containing suitable sorbent particles or beads, such as, for example, the alkali-impregnated alumina sorbent particles disclosed in U.S. Pat. No. 4,755,499 entitled SORBENT FOR REMOVING NITROGEN OXIDES, SULFUR OXIDES AND HYDROGEN SULFIDE FROM GAS STREAMS, issued Jul. 5, 1988, Lewis G. Neal, et al., inventors, and assigned to NOXSO Corporation, the same assignee as the present invention; this patent is also incorporated herein by reference and these sorbent particles are referred to in the art and hereinafter and in the appended claims as the NOXSO Sorbent or the NOXSO Sorbent Particles. Adsorber 14 has a fluidizing grid 15 for fluidizing the NOXSO Sorbent. The SO.sub.x and NO.sub.x are adsorbed on the surfaces of the NOXSO Sorbent, contained in the fluid bed adsorber 14, and thereby removed from the flue gas stream 12 which is discharged to the atmosphere from the adsorber 14 via line 70.
The loaded or saturated NOXSO Sorbent, i.e. the NOXSO Sorbent having adsorbed the NO.sub.x and SO.sub.x from the flue gas, is subsequently transported to a staged, fluid bed heater 18 wherein the temperature of the NOXSO Sorbent is raised above 532.degree. C. (1000.degree. F.) using a stream of hot air 20 supplied by hot air heater 22 into which air heater a stream of ambient or combustion air 24 and a suitable fuel or fuel stream 26, e.g., natural gas, enter; the hot air heater 22 also produces combustion products which enter the hot air stream 20. As the temperature of the NOXSO Sorbent rises in the fluid bed heater 18 and passes through about 350.degree. C., the NO.sub.x that was adsorbed by the loaded NOXSO Sorbent is removed or stripped from the NOXSO Sorbent, mixes with the hot air stream 20 flowing through the fluid bed heater 18 and is carried away in an off-gas stream having a temperature of about 165.degree. C.; the off-gas stream 30 passes through cyclone 28 and is transported to and mixed with the power plant (such as the oil-fired or coal-fired power plant noted above) combustion air stream (not shown) where the NO.sub.x is destroyed in the power plant combuster.
The hot NOXSO Sorbent with the NO.sub.x removed therefrom, but with the SO.sub.x still adsorbed thereon, is transferred from the sorbent heater 18 into a moving bed regenerator 32 via line 34. In the moving bed regenerator 32, the NOXSO Sorbent is contacted with a suitable regenerant gas stream 36. The regenerant gas 36 reacts with the SO.sub.x adsorbed by the NOXSO Sorbent, removes or desorbs the SO.sub.x from the NOXSO sorbent, and produces elemental sulfur. Off-gas stream 38 containing elemental sulfur is transported into a sulfur condenser and mist eliminator 45 wherein a steam stream 42, water stream 44 and elemental sulfur stream 46 are produced. A stream 40 from the sulfur condenser and mist eliminator 45 is returned to regenerator 32.
The regenerated NOXSO Sorbent, or NOXSO Sorbent Particles, i.e. NOXSO Sorbent with the SO.sub.x and NO.sub.x removed, is transported via stream 48 past valve 50 to a staged, fluid bed sorbent cooler 52, where the NOXSO Sorbent is contacted with atmospheric air supplied via line 54 to reduce the temperature of the NOXSO Sorbent to about 120.degree. C. (250.degree. F.). The heated atmospheric air 56, having passed through the sorbent cooler 52 and having its temperature raised to about 450.degree. C. due to its heat exchange with the heated NOXSO Sorbent in the sorbent cooler 52, is transported to gas heater 22 where the temperature of the hot air stream 56 is increased by the hot air heater 22 and used to supply, at least in part, the hot air 20 supplied to the fluid bed heater 18 to remove the NO.sub.x from the NOXSO Sorbent as described above; it will be noted that the temperature of the hot air stream 56 leaving the sorbent cooler 52 (about 450.degree. C.) is higher than the air temperature of the hot air stream leaving the fluid bed heater 18 (about 165.degree. C.) and carrying away the removed NO.sub.x.
Cooled regenerated NOXSO Sorbent is transported via line 58 by air from line 54 to a pneumatic lift line 60 which lifts the NOXSO Sorbent into a cyclone separator 62 via stream 64. Cyclone separator 62 separates stream 64 into a stream of air 66 and a stream of regenerated NOXSO Sorbent or NOXSO Sorbent particles 68. Regenerated NOXSO Sorbent 68 enters the fluid bed adsorber 14.
In brief summary, it will be understood that in the NOXSO Process, as taught in the 4,798,711 patent, NO.sub.x and SO.sub.x are removed from the flue gas stream 12 by the NOXSO Sorbent in the fluid bed adsorber 14 to produce a stream of NO.sub.x and SO.sub.x substantially free flue gas 70 and thereafter the fluid bed heater 18, moving bed regenerator 32 and fluid bed sorbent cooler 52 regenerate the NOXSO Sorbent particles which adsorbed the SO.sub.x and NO.sub.x in the fluid bed adsorber 14 whereafter the regenerated NOXSO Sorbent Particles are transported via line 58 to the pneumatic lift line 60 for return through the cyclone separator 62 to the fluid bed adsorber 14 whereafter the NOXSO Process is repeated.
While the NOXSO Process has been proven to be successful in removing NO.sub.x and SO.sub.x from gas, e.g. flue gas, there are several disadvantages to the destruction of the NO.sub.x in the boiler of the power plant producing the flue gas, namely:
(i) the NO.sub.x destruction efficiency is specific to the individual boiler making it necessary to generate a large data base of NO.sub.x destruction data for the successful practice of the NOXSO Process in different boilers; PA1 (ii) in multi-boiler applications, operating flexibility is reduced by requiring that the NO.sub.x recycle stream be returned to an operating boiler with adequate capacity to accept the NO.sub.x recycle stream into the combustion air system of the boiler; PA1 (iii) inclusion of the NO.sub.x recycle stream in the combustion air of the power plant boiler requires careful integration of the NOXSO Process with the power plant to assure that neither has an adverse effect on the operation of the other; PA1 (iv) net NO.sub.x removal efficiency of the NOXSO Process is significantly influenced by the NO.sub.x destruction efficiency of a specific boiler; modifications to the boiler to increase NO.sub.x destruction efficiency typically meet with resistance from the boiler operator; PA1 (v) the concentration of NO.sub.x in the off-gas stream from the fluid bed heater 18 is so low or small that it is relatively technically inefficient and commercially infeasible to feed such off-gas stream carrying the removed NO.sub.x directly into NO.sub.x reduction or destruction apparatus such as NO.sub.x reducing burners, SCR (Selective Catalytic Reduction Apparatus), SNCR (Selective Non-Catalytic Reduction Apparatus) and the like known for reducing or destroying NO.sub.x, or to feed such off-gas stream carrying the removed NO.sub.x directly into a device or apparatus which produces or generates from the removed NO.sub.x a marketable nitrogen by-product, e.g. nitric acid, calcium nitrate or ammonium nitrate.
Accordingly, it has been found that there is need for an improved process and apparatus generally, and in particular a need for an improved NOXSO Process and improved apparatus for practicing the NOXSO Process, for removing at least NO.sub.x from a gas, such as flue gas, wherein the NO.sub.x destruction, or reduction, does not experience the above-noted disadvantages of destroying or reducing the NO.sub.x in the boiler of a power plant and wherein the concentration of NO.sub.x in an off-gas stream carrying NO.sub.x removed from sorbent such as the NOXSO Sorbent is sufficiently high that it is technically efficient and economically feasible to feed the NO.sub.x off-gas stream directly into NO.sub.x destruction or reduction apparatus such as the above-noted NO.sub.x reducing burner, SCR, SNCR and the like, or into a device or apparatus which generates a marketable nitrogen by-product from the removed NO.sub.x.